This invention relates to multilayer ceramic printed circuit boards including co-fired passive components. More particularly, this invention relates to multilayer ceramic printed circuit boards including embedded resistors and method of making them.
Low temperature multilayer ceramic circuit boards are known that are suitable for use with low melting temperature conductive metals, such as silver, gold and copper. They have a low thermal coefficient of expansion (TCE) and thus they may be compatible with silicon or gallium arsenide. The ceramic circuit boards are made from glasses that can be fired at low temperatures, i.e., less than 1000xc2x0 C. The multilayer circuit boards are made in known manner by admixing suitable glass powders with an organic vehicle, including resin, solvents, dispersants and the like, and casting the resultant slurry into a thin tape called green tape. A circuit may be screen printed onto the green tape using a conductive ink formulation comprising a conductive metal powder, an organic vehicle and a powdered glass, usually the same or a similar glass to that used to make the green tape.
When more than one green tape is to be stacked, via holes are punched into the tape which are filled with a via fill ink, also made with a conductive material, an organic vehicle and a glass, which will provide electrical contact between the circuits on the various green tape layers. The patterned green tapes are aligned and compressed or laminated under pressure prior to firing.
More recently, the multilayer ceramic circuit boards have been adhered to a metal support substrate to increase the strength of the multilayer board. The support board has a metal core that is coated with a bonding glass that adheres the ceramic tapes to the support substrate during co-firing. The use of a bonding glass has another advantage in that it greatly reduces the shrinkage of the green tapes during firing in the x and y dimensions, so that most of the shrinkage occurs only in the z, or thickness, dimension. The glasses used for the green tapes must have a TCE matched to the metal support however, to prevent delamination or cracking of the fired glass. Mixtures of crystallizable and non-crystallizable glasses, optionally including inorganic fillers, are also known that have the desired TCE characteristics.
Up to the present time, when multilayer ceramic circuit boards are to include passive components such as resistors or capacitors, discrete components have been mounted to the top surface of the fired boards using solder or epoxy adhesives to adhere the components to the multilayer ceramic. The incorporation of these discrete components increases the number of steps needed to make them, i.e., the components must be aligned and adhered to the ceramic multilayer board, and connected to a source of power. Further in order to accommodate a number of discrete devices, the multilayer boards have to be large. Thus the costs of making such boards is high.
It would be advantageous to be able to screen print passive components onto multilayer, low temperature co-fired ceramic circuit boards because the packing density can be increased, reducing the size and cost of the packaging. Using the recently developed low firing temperature glasses and a metal support board that reduces shrinkage in the x and y dimensions, screen printing of such components to tight tolerances and high precision placement becomes feasible. Further, because fewer interconnects need to be made, reliability would also be improved.
In a copending application filed concurrently herewith, the present inventors have interleaved two types of green tapes so that large numbers of green tapes can be stacked without shrinkage in two dimensions.
Thus it would be highly desirable to develop appropriate ink compositions that can be screen printed onto green tape layers to form embedded resistors to tight tolerances with high precision placement.
We have found a method of making thick film resistor ink compositions based on ruthenium oxide (RuO2) and appropriate glasses that sinter at low temperatures, e.g., 850-900xc2x0 C., together with suitable organic vehicles. The resistor inks can be screen printed onto known low firing temperature green tape stacks, preferably supported on a metal support substrate and covered with one or two green tapes to produce embedded resistors having a wide range of resistor values and thermal coefficient of resistance (TCR) values. Small amounts of barium titanate can also be added to the resistor inks to adjust TCR values. The resistors can be connected to a source of power by means of a conductive layer screen printed on top of the fired, supported green tape stack. After printing the resistors and other circuitry, the multiple green tape layers are aligned, laminated together, applied to a metal support substrate via a bonding glass, and co-fired in air at a temperature of about 700-900xc2x0 C. The resultant embedded resistors are stable and reliable.